Solar cells, photodiodes, radiation detectors and other optoelectronic devices utilize light-sensitive or radiation energy-sensitive materials (hereinafter energy-sensitive materials) that are capable of converting incident visible light or other radiation energy (e.g. x-rays, alpha rays and beta rays) into electrical energy. Light transmissive, electrically-conductive oxide films are frequently used in combination with these energy sensitive materials to allow incident light or radiation energy to pass through the film and strike the energy-sensitive material. In another application, such as with liquid crystal displays, electrically-conductive oxide films are used as light transmissive electrodes. In yet another application, light transmissive, electrically-conductive oxide films can be used as transparent resistors, for example, in window heaters or defrosters.
To be useful in the above applications, an oxide film must demonstrate a combination of features including transparency to visible light and electrical conductivity. Examples of well known and useful transparent conductive oxides are ZnO, In.sub.2 O.sub.3 /SnO.sub.2, In.sub.2 O.sub.3, CdSnO.sub.4, SnO, and SnO.sub.2.
It is known that properties of oxide films, particularly resistivity, can be enhanced by the inclusion of certain dopants. U.S. Pat. No. 4,623,601 (Lewis et al.) describes transparent, electrically-conductive zinc oxides doped with hydrogen or Group III elements. Films produced from these oxides exhibit reduced resistivity, and thereby are said to enhance the overall efficiency of photoconductive devices in which they are used.
However, researchers have also found that some of the enhanced conductive properties of these films are not stable over time. For instance, it has been shown that indium tin oxide (ITO) films deposited by reactive sputtering in the presence of H.sub.2 and/or O.sub.2 at relatively low temperatures (around 100.degree. C. to 200.degree. C.) suffer an increase in sheet resistivity over time when exposed to laboratory air at room temperature. G. L. Harding and B. Window, DC Magnetron Reactively Sputtered Indium-Tin-Oxide Films Produced Using Argon-Oxygen-Hydrogen Mixtures, 20 Solar Energy Materials 375 (1990).
Vasanelli el al., infra, report finding initial resistivity values of zinc oxide films deposited in H.sub.2 -Ar mixtures within the range from 10.sup.-3 to 10.sup.1 ohm cm, while zinc oxide films deposited in pure Ar had resistivities of about 10.sup.2 to 10.sup.4 ohm cm. Vasanelli et al. further report that when samples of their zinc oxide films were exposed to air at elevated temperatures (200.degree. C.), the resistivity of the films increased. The resistivity of film samples deposited at higher H.sub.2 concentrations suffered the most severe resistivity increases. L. Vasanelli, A. Valentini and A. Losacco, Preparation of Transparent Conducting Zinc Oxide Films by Reactive Sputtering, 16 Solar Energy Materials 91, 96-97 (1987).
To achieve greater stability, Vasanelli et al. performed post-deposition annealings of the oxide films. They report finding improved stability of zinc oxide films over time after exposing samples of the films to N.sub.2 or H.sub.2 at 200, 300, and 400.degree. C. The greatest improvements were gained at the highest temperatures. Vasanelli et al. at 97.
U.S. Pat. No. 5,135,581 (Tran et al.) describes the achievement of increased stability without a post-deposition annealing step by sputtering a target in an atmosphere of fluorine-containing gas and a stabilizing gas such as H.sub.2 or H.sub.2 O. Tran et al. describe a process for making transparent, conductive oxide films having decreased resistivity (10-4 to 10.sup.-2 ohm cm) and improved stability over time. Useful conductive oxides listed by Tran et al. are: ZnO optionally doped with Al, In, Ga, or B; SnO doped with at least one of F or Sb; indium tin oxide (In.sub.2 O.sub.3 /SnO.sub.2); CdSnO.sub.4 ; TiO.sub.2 doped with F; and SnTiO.sub.3.
U.S. Pat. No. 4,146,657 (Gordon) teaches that tin oxides doped with fluorine may be formed by forming a reactive vapor which will produce, upon heating, a compound having a tin-fluorine bond, and bringing this vapor to a heated surface, on which a fluorine-doped tin oxide deposits. Col. 3 lines 58-63. However, by this process, the heated surface is typically heated to temperatures of about 400.degree. C. to 600.degree. C. Col. 4 lines 34-35.
Exposing transparent conductive oxide films deposited on energy-sensitive materials to high temperatures (greater than about 400.degree. C.), has several disadvantages. Disadvantages are that exposing the devices which utilize these oxide films (e.g. photosensors and radiation detectors) to high temperatures can cause migration of the oxide into the light sensitive material of the device, or migration of metals and dopants between layers of the energy sensitive film. Either of these effects can impair or destroy the device's ability to function. Furthermore, the high temperature process limits the useful substrates to those which will not undergo degradation at these elevated temperatures. Tran et al. '581 col. 1-2.